A lithium ion secondary battery is a secondary battery that has high charging/discharging capacity and that can carry out high output. The lithium ion secondary battery is currently used mainly as a power source for a portable electronic device, and is further expected to be used as a power source for an electric automobile, which is assumed to prevail in the future. The lithium ion secondary battery has, at a positive electrode and a negative electrode, an active material that can insert and desorb lithium (Li). The lithium ion secondary battery operates when the lithium ions move in an electrolysis solution provided between the electrodes.
The lithium ion secondary battery is desired to maintain the discharging capacity even after repeating charging and discharging. However, the charging/discharging cycle lifespan of the lithium ion secondary battery becomes short as the electrode active material and the electrolysis solution gradually react thus decomposing the electrolysis solution.
Various considerations have been made to suppress such decomposition of the electrolysis solution. For example, Patent Literature 1 discloses mixing a chemical compound, which contains polysiloxane, perfluoropolyether, perfluoroalkane, and derivatives thereof, in the electrolysis solution, and forming a coated layer of the chemical compound on the positive electrode or the negative electrode. Such chemical compounds are chemical compounds that have smaller surface tension than the electrolysis solution and that are insoluble in the electrolysis solution, and thus the coated layer is formed on the electrode in the battery when the battery is assembled. Patent literature 2 discloses covering a lithium ion conductive polymer compound including a polyethylene glycol unit on the active material consisting tin oxide or composite tin oxide.
However, the coated layer may detach from the electrode as the number of cycles becomes greater in a battery in which the coated layer is attached to the electrode as described in Patent Literature 1, and it is troublesome to cover the polymer compound on the active material itself as described in Patent Literature 2.
Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-265609
Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 10-312803